In some audio speakers, a Motion Feed Back (“MFB”) circuit is included to improve the sound quality of the speaker. The MFB circuit detects the operating state of a vibrating diaphragm through an electrical signal conveying audio information (hereinafter referred to as an “audio signal”) that is inputted to a speaker. The MFB circuit controls the diaphragm based on the detection result. In this manner, the distortion of sound, which is likely to occur especially in a low tone region, can be canceled. Therefore, it is sometimes mistakenly assumed that the MFB circuit is effective to be utilized in a small-sized speaker in which reproduction in a low tone region is difficult.
For example, the following five references with regard to a MFB circuit are known: Japanese Patent Laid-Open No. Sho 52-79644, Japanese Patent Laid-Open No. Sho 53-12319, Japanese Patent Laid-Open No. Sho 53-12320, Japanese Patent Laid-Open No. Sho 53-12321, and Japanese Utility Model Laid-Open No. Sho 57-96589. In these references, the operating state of the diaphragm is detected by detecting the variation of an electrostatic capacity formed between electrodes. More specifically, an electrode (hereinafter referred to as “movable electrode”) is fixed to a diaphragm, or to an electromagnetic coil which is referred to as a “voice coil bobbin” that causes the diaphragm to vibrate, and another electrode (hereinafter, referred to as “fixed electrode”) is fixed so as to face the movable electrode. An electrostatic capacity, which varies by the movable electrode moving relative to the fixed electrode, is detected by a detector and is converted into an electrical signal (hereinafter, referred to as “detection signal”) in a converter circuit to be outputted. Further, the detection signal and the audio signal are then compared with each other by a comparison device (a CPU, for example), and then the operation of the diaphragm is appropriately controlled on the basis of the compared result, i.e., the difference between the output level of the detection signal and the output level of the audio signal.
However, the electrostatic capacity that is formed between the electrodes is very small, for example, from several pF (picofarad) to several hundred pF. Therefore, the electrostatic capacity is affected and varied by noise such as a very small amount of an electromagnetic wave or static electricity. For example, a diaphragm is commonly structured to be vibrated by an excitation effect between a voice coil bobbin, an iron core which is inserted into the voice coil bobbin and referred to as a center pole, and a magnet which generates a magnetic flux passing through the voice coil bobbin and the center pole. However, the electrostatic capacity between the electrodes is affected and varied by an exciting current flowing through the voice coil bobbin. Further, some of electronic components which are incorporated into the speaker emit an electromagnetic wave although it may be weak, and the electrostatic capacity may be varied by the electromagnetic wave transmitting to the electrodes. Further, the electrostatic capacity between the electrodes may be affected by friction accompanied with mechanical phenomena such as the vibration of components which are incorporated in the speaker, static electricity caused by various electromagnetic phenomena in the inside and the outside of the speaker, electromagnetic waves which are outputted by electronic equipment installed around the speaker, or the like (hereinafter, referred to as “disturbance noise”). Thus, in the above-mentioned references, the electrostatic capacity varies and the electrostatic capacity formed between the electrodes is unable to be accurately detected.